Compositions comprising phosphoric acid and aluminum metal are well known for use in protecting metallic surfaces such as ferrous surfaces from corrosion. In such coating compositions, particulate metallic material, such as aluminum flake and/or powder, is combined with a phosphoric acid bonding solution to form a coating composition which is then applied to the metallic surface being treated. After application of the coating to the substrate, it may be heated to a first temperature, generally upwards of 500° F. (260° C.), until the coating is rendered essentially water insoluble. Then the coated surface may be cured at a second temperature, generally above 1000° F. (538° C.) to form the final protective coating.
A problem which arises in this coating process is that when the particulate metallic material is combined with the phosphoric acid bonding solution, the acidic bonding solution can react with the metallic material. Such reactions can be very violent, causing the flake and/or powder to burn or even explode, or less violent, simply resulting in the conversion of the metallic material into various salts. In either case, such reactions interfere with the formation of suitable protective coatings.
Allen U.S. Pat. No. 3,248,251 describes coating compositions consisting essentially of a slurry of solid inorganic particulate material (such as aluminum) in an aqueous acidic bonding solution containing dissolved metal chromate, dichromate or molybdate, and phosphate. It was found that the addition of chromates or molybdates to the acidic bonding solution effectively passivated the solution toward aluminum and inhibited the oxidation of metallic aluminum, allowing particulate aluminum to be combined with the bonding solution without the undesirable chemical reaction between the acidic solution and the aluminum. These so-called Allen coatings have been successfully used to provide high quality coatings which protect ferrous metal alloy surfaces from oxidation and corrosion, particularly at high temperatures.
While chromates and molybdates have been used successfully to reduce the reactivity of the aluminum in such coating compositions, the use of chromates and molybdates has become a problem because of environmental considerations. Chromates and molybdates are generally considered to be toxic substances. Hexavalent chromium is rated as a carcinogen. It is therefore desirable to avoid the use of solutions of such salts, or at least to reduce their use. For this reason, it has become desirable to develop a phosphate/aluminum coating composition which requires little or no chromates or molybdates to control the reactivity between the acidic phosphate bonding solution and the particulate aluminum added thereto. The coating compositions should protect ferrous metal alloy surfaces from the oxidation and corrosive environmental conditions, especially at high temperatures, at least as well as the Allen-type coatings.
Some efforts have been made to overcome the environmental problem associated with chromates and molybdates. Stetson et al. U.S. Pat. No. 5,242,488 describes a coating composition for ferrous alloys which does not require either chromates or molybdates to control the reaction between the bonding solution and the powdered aluminum. The composition consists essentially of a slurry mixture of a bonding solution and aluminum powder. The bonding solution consists essentially of water, phosphoric acid (H3PO4), and aluminum ions in solution in an amount sufficient to substantially equilibrate the bonding solution with respect to aluminum metal pigment. The bonding solution component of the coating composition requires sufficient aluminum ions in solution so that it is substantially equilibrated with respect to aluminum metal pigment, i.e., that the amount of aluminum ions in solution be substantially at the saturation point and therefore, essentially inert with respect to any subsequent addition of aluminum metal pigment.
Stetson discloses using magnesium (either MgO or MgCO3) to at least partially neutralize the aqueous phosphoric acid mixture, either before or after equilibration of the mixture with aluminum. A more recent patent to Stetson et al., U.S. Pat. No. 5,279,649, describes substantially the same compositions to which V2O5 has been added to produce vanadate ion, adding another inhibitor to the aluminum equilibrated mixture. Further, Stetson et al. U.S. Pat. No. 5,279,650 discloses a seal coating composition of the coating disclosed in the '649 patent which also contains iron oxide (Fe2O3) powder. All three coating compositions are designed to avoid the use of the chromium and molybdenum ions and require the bonding solution to be equilibrated with respect to further additions of aluminum. The addition of V2O5 demonstrates the addition of a toxic substance, listed on the OSHA extremely hazardous substance list.
Environmentally friendly coating compositions are described in commonly assigned Mosser et al. U.S. Pat. No. 5,478,413, which discloses an aqueous solution of phosphoric acid, one or more sources of magnesium ions, and a source of aluminum ions and/or sources of zinc and borate ions. The amount of aluminum in solution is in an amount less than that necessary to reach the saturation point, thus, less than the amount necessary to equilibrate the bonding solution with respect to additional aluminum. The bonding solutions are non-reactive with respect to particulate aluminum for at least one hour.